The present invention relates to cathode ray tubes (CRTs). A CRT is a device in which high-energy electron beams are projected in a vacuum onto a screen which contains a layer of a luminescent material. The direction and intensity of the electron beams in CRTs are controlled to produce visible images at a predetermined location at the screen. For general description of CRTs, their structure and use, see, for example, McGraw-Hill Encyclopedia of Science and Technology, vol. 2, pp 666-672 (5th edition, 1982), which is hereby incorporated by reference.
In the most general terms, a cathode ray tube comprises an envelope which serves as a vacuum enclosure; an electron gun, which is a source of the electron beams and which also includes means for controlling the direction of the electron beams, and a luminescent layer.
X-rays are also created inside CRTs when the electron beams bombard the surface of a material with high atomic weight, e.g. the glass screen of the envelope. For brief description of the X-ray formation and nature, see McGraw-Hill Encyclopedia of Science and Technology, vol. 14, pp 758-761 (5th edition, 1982), which is also hereby incorporated by reference. To contain the harmful X-rays inside a cathode ray tube, CRT envelopes typically are made of materials with good X-ray absorbing properties, such as, for example, lead-containing glass. Lead is a highly efficient X-ray absorber, and, therefore, it is very effective in containing the X-rays. However, an interaction between the lead-containing glass and the high-energy electron beams leads to molecular damage in the lead-containing glass in the area where the electron bombardment occurs. The damage manifests itself as a color change called glass browning. See, for example, Ishiyama et al., Browning of Glass by Electron Bombardment, Proceedings of Tenth International Congress on Glass (1974), also hereby incorporated by reference. The X-rays also contribute to glass browning, but their contribution is much smaller.
Since CRTs are widely used in televisions, computers, and various other numeric and imaging devices to create visible images, glass browning is a very serious problem for CRT users and producers. Browning destroys the true nature of colors displayed by CRTs and directly reduces the functionality of the CRT devices. Glass browning is especially detrimental to those CRT users who depend on true colors: art departments, photographers, graphic designers, desktop publishers, and others.
The glass browning problem has plagued the CRT industry for many years. The problem, however, became more pronounced as higher voltages are used in CRTs to enhance picture quality. Currently, the only alternative to coping with glass browning is the use of lead-free glass in the CRT envelopes. To provide satisfactory X-ray containment, large quantities of barium and strontium oxides are currently used in CRT envelopes. Barium and strontium oxides are much more expensive than lead oxide, and must be incorporated in higher concentrations vis-a-vis lead in order to provide comparable X-ray absorbing properties. Further increase in CRT voltage will strain capacity of existing lead-free CRT envelopes to contain X-rays. The use of a lead-containing glass is preferred due to the unique ability of lead to block X-rays and because it is economical. Moreover, well-developed processes for manufacturing lead-containing glass panels for CRTs are both available and inexpensive.
For the foregoing reasons, there is a definite and unfulfilled need for an inexpensive CRT envelope that absorbs X-rays and yet is not subject to glass browning.